Download Gotzek, D., H. J. Axen, A. V. Suarez, C. S.H., and D. D. Shoemaker

Journal of Insect Conservation 8: 15–25, 2004.
# 2004 Kluwer Academic Publishers. Printed in the Netherlands.
15
Impact of the invasive crazy ant Anoplolepis gracilipes on Bird Island,
Seychelles
J. Gerlach
University Museum of Zoology Cambridge, Department of Zoology, Downing Street, Cambridge CB2
3EJ, UK (e-mail: [email protected])
Received 26 March 2003; accepted in revised form 12 December 2003
Key words: Anoplolepis gracilipes, Ant, Formicidae, Invasion, Management, Seychelles
Abstract
The crazy ant (Anoplolepis gracilipes) invaded Bird island, Seychelles, in the 1980s. In 1997, its range
expanded and population densities increased. The impacts of this change were studied in 2001 using a
combination of arthropod collecting methods. The ant population excluded larger invertebrates (principally
the large ant Odontomachus simillimus and the crabs, principally Ocypode spp.). Cockroaches, however,
remained abundant in ant-infested areas and tree-nesting birds (Lesser Noddy Anous tenuirostris) appear to
be able to breed successfully in the presence of the crazy ant. The ants are only abundant in areas of deep
shade which provide cool nesting areas, yet enabling them to forage in the open when ground temperatures
fall. The expansion of the ants was correlated with the regeneration of woodland on the island.
Recommendations are made for the management of the woodland which may reduce the impacts of the
crazy ant.
Introduction
The threat from alien invasive species is widely
perceived as one of the major contributions to
present-day high-extinction rates, affecting an estimated 30% of threatened birds, 15% of threatened
plants and 10% of threatened mammals (HiltonTaylor 2000). Invasive predatory ant species may
be particularly problematic (Williams 1994; Moller
1996; Tsutsui and Suarez 2003) as exemplified by
the spread of the ants Linepithema humile (Mayr),
Pheidole megacephala Fabricius, Solenopsis
wagneri Santschi (¼S. invicta) and Wasmannia auropunctata (Roger) in the North America, the
Galapagos and Hawaii islands (Haskins and
Haskins 1965; Clark et al. 1982; McGlynn 1999)
and widespread invasions by the crazy ant
Anoplolepis gracilipes (F. Smith) (¼A. longipes).
This tropical tramp species has been widely introduced and is now found throughout much of
Africa, Asia, America and Australia (Dorow
1996). This species has been recorded as excluding
other ant species (Greenslade 1971) and recently
has been implicated in a population crash in the
red crab Gecarcoidea natalis on Christmas Island
(O’ Dowd et al. 1999). It has been listed as one of
the 100 ‘worst invasive alien species’ (Lowe et al.
2001).
A. gracilipes is believed to have been introduced
to Mahe island in the Seychelles islands in about
1962, remaining in the Maldive area of the island
until 1970, and then having colonized most lowland areas by 1975 (Haines and Haines 1978b).
Praslin island was colonized in 1975; the population appeared to have died out by the late 1970s
without intervention (Haines and Haines 1978b).
However, it is now known to be well established on
the island (Dorow 1996) and it is now present on
nine islands (Mahe, Anonyme, St. Anne, Praslin,
Petite Soeur, Marianne, Felicite, La Digue and
16
Figure 1. Central islands of the Seychelles group, with A. gracilipes invaded islands shaded.
Bird island; Gerlach 2003) (Figure 1). The impact
of A. gracilipes on Mahe in the 1970s resulted in
significant declines in the abundance of other ants,
several other insect groups, millipedes and spiders
(Lewis et al. 1976; Haines and Haines 1978a).
The most recently reported invasion of
A. gracilipes in Seychelles has been on Bird
island. While the species colonized the island in
the 1980s, probably in building materials imported
from Mahe island, it remained restricted to buildings until 1997 (Robert 1998a; Feare 1999). In that
year, the ant spread into the island’s colony of
nesting sooty tern (Sterna fuscata Linnaeus), occupying 16% of the colony’s range in 1998 and 50% in
1999 (Robert 1998c, 1999a). This caused the failure
of tern breeding in 1999 due to the ants swarming
over the ground, disturbing the chicks and causing
them to shake their feet constantly. This is normally a behaviour used to dislodge ticks but the
numbers of ants resulted in constant repetition of
the movement, leading to excessive energy expenditure, reduced feeding, and ultimate exhaustion
and death (Robert 1999b). Attempts at control
have included a range of chemical baits and sprays.
Currently control relies on cypermethrin-based
insecticidal sprays combined with burning to clear
ants from the area used by the sooty tern colony
(Robert 1999a). This has successfully prevented the
ant from occupying the area of the tern colony
since 2000.
Bird island is the most northerly of the
Seychelles islands and is a coral island, resting
on the edge of the Seychelles plateau. As a small
(101 ha) coral island, all its habitats can be
described as coastal, with a strong maritime influence on all areas. Originally the island was probably a mixture of open habitat used by the nesting
terns and Pisonia grandis R.Br. woodland. Most of
the habitat of the island was modified in the 1900s
by the establishment of a coconut (Cocos nucifera L.)
17
plantation. This is now abandoned and P. grandis
woodland is regenerating. The vegetation includes
many introduced species, few of which are significant components of any habitat. The exception is
Carica papaya L. which is widely dispersed (and
increasing locally) by birds. There is no significant
agriculture or gardening and habitat management
and chemical input are restricted to seasonal burning and insecticide spraying of one area to maintain
open habitat for the sooty tern colony. This has no
detectable impact on other parts of the island. Bird
Island has been visited by biologists on several
occasions since 1905. Available data on the island’s
biota are largely concentrated on the sooty tern
colony but scattered observations exist (Fryer
1909; Benoit 1978; Feare 1979; Stoddart and
Fosberg 1981). Surveys of the island’s ecosystems
were made in April and October 2001 and October
2002. These complement the data accumulated on
the spread of the crazy ants (Gerlach 2003) providing an evaluation of the impact of this species on
Bird island.
Methods
Distribution of the crazy ant
Since 1997, the distribution of the crazy ant has
been monitored by residents of the island and during attempts to control the species and eradicate it
from the sooty tern colony. These observations
were compiled to provide an approximate rate of
expansion. The island was divided into a grid of
40 squares, each measuring approximately 150 m,
using the runway and main path for orientation
and measured by pacing. In each square 20 randomly positioned 1 m2 quadrats were studied. In
these quadrats, the number of ants active on the
surface of the leaf litter at mid-day was counted
and the canopy cover estimated visually. Counting
ants at mid-day provided a comparative measure of
activity and abundance for each square. The square
was observed from a distance of 2 m to ensure
minimal disturbance to the ants. The association
between crazy ant numbers and habitat factors was
analysed using a multiple analysis of covariance of
canopy cover (leaf litter depth was found to be
directly correlated (F ¼ 10.2, P < 0.0001) to canopy
cover and was excluded from analysis) and
minimum distance from the sea. In each grid
square the presence or absence of potential nest
sites (logs, piles of leaves or rocks or man-made
structures) was recorded.
Biodiversity
Arthropod diversity on Bird island was surveyed
on 4th and 5th April 2001, 21–23rd October 2001
and 20–22nd October 2002. During each visit,
10 samples of leaf litter, each covering 1 m2, were
collected from an A. gracilipes free area and an
infested area (points A and B in Figure 2). These
sites differed primarily in the presence or absence
of the crazy ant. Both were areas of woodland,
details of comparisons of vegetation and tree
cover are given in Table 3. All leaf litter types
were sampled, for samples from coconut woodland
the superficial layers of leaves were removed and
the partially decomposed sub-surface leaves used.
The superficial layers did not support invertebrate
life as revealed by careful examination of leaves
outside of the sample quadrats. These samples
were sieved and placed in Tullgren funnels for
24 h. Sweep netting of 10 m2 was also carried out
in these two areas. In October 2002, no A. gracilipes-free woodland could be found, and 20 leaf
litter samples were collected from randomly
selected grid squares. For each sample, the invertebrate composition, litter depth, canopy cover and
distance from the sea were recorded. The association between invertebrate numbers and crazy ants
was analysed using a multiple analysis of covariance of crazy ant numbers, canopy cover and
minimum distance from the sea.
The numbers of nesting terns in ant-invaded and
ant-free areas were investigated by recording the
number of active lesser noddy (A. tenuirostris
(Temminck)) nests on P. grandis trees. Ten trees
with estimated heights of 10–45 m were examined
in the ant invaded area and adjacent to it.
Observations were made between 10:00 and 12:00 h
for comparative data. Trees infested with ants were
easily identified by the trials of ants on the trunks,
trees without ants were double checked by disturbing the leaf litter around the base of the tree and
fissures in the bark. If no ants could be located by
this method, the tree was concluded to be
ant-free.
18
Crazy ant behaviour
The behaviour of the crazy ant visible on the surface of the ground was observed at irregular times
throughout the day by counting the maximum
number of workers foraging on the surface at
any one time in a 1-min observation period.
Observations were made between 11:00 and 13:00 h
in each grid square. In one grid square (point A),
these observations were repeated hourly from 6:00
to 18:00 h. In each case, the square was observed
from a distance of 2 m, where possible from a rock
or area of open ground, to minimise disturbance to
the ants. Night-time observations required close
approach to the ants and disturbance could not
be prevented, thus reliable comparative night-time
observations could not be obtained.
Results
Distribution of the crazy ant
The ant has spread from the buildings on the west
of the island into the central woodland areas and
the open ground of the tern colony in the north. In
2001, 70 ha of the island were occupied by the ant,
with the southern and eastern parts of the island
remaining ant free (Figure 2). By October 2002, the
ant had spread over the whole island. A significant
association was found between the numbers of
ants, distance from the sea and canopy cover
(Table 1; Figure 3). The effect of canopy cover
was observed in the activity data but could not be
accurately analysed in leaf litter data as the heat
from the lamps used in the Tullgren funnels killed
all crazy ant individuals collected (but did not have
any such effect on other taxa). The ant was only
observed during the middle of the day where
canopy cover exceeded 75%, although disturbance
or observations at other times revealed their presence in all grid squares in 2002. Potential nest sites
were found to be uniformly distributed across the
island, with no association with areas of high or
low crazy ant abundance.
Figure 2. Habitats and the crazy ants on Bird island: (a) habitats
on Bird island (habitat comparison points A and B marked); (b)
crazy ants range expansion since 1997 (based on reports from
island residents).
Crazy ant behaviour
The crazy ant is crepuscular. Activity on the surface of the ground was observed between 6:00 and
19
Table 1. Results of statistical testing (MANCOVA) of the effect of habitat and Anoplolepis gracilipes abundance on the abundance of
invertebrates
Taxon
Variable
df
Seq. SS
Adj. SS
Adj. MS
F
P
Anoplolepis gracilipes (F. Smith)
Cover
Distance to sea
Error
Total
Cover
Distance to sea
Anoplolepis
Error
Total
Cover
Distance to sea
Anoplolepis
Error
Total
Cover
Distance to sea
Anoplolepis
Error
Total
Cover
Distance to sea
Anoplolepis
Error
Total
Cover
Distance to sea
Anoplolepis
Error
Total
Cover
Distance to sea
Anoplolepis
Error
Total
Cover
Distance to sea
Anoplolepis
Error
Total
Cover
Distance to sea
Anoplolepis
Error
Total
Cover
Distance to sea
Anoplolepis
Error
Total
Cover
Distance to sea
Anoplolepis
Error
Total
6
2
31
39
1
1
1
36
39
1
1
1
36
39
1
1
1
36
39
1
1
1
36
39
1
1
1
36
39
1
1
1
36
39
1
1
1
36
39
1
1
1
36
39
1
1
1
36
39
1
1
1
36
39
1213.34
315.20
365.23
1893.78
19.39
38.00
8.56
318.44
384.40
27.06
7.41
4.03
81.61
120.09
32.08
15.23
3.29
76.67
127.26
29.80
14.82
3.00
76.01
134.14
0.00
3.91
2.59
34.47
40.98
29.06
5.30
4.00
66.17
104.51
35.80
4.17
3.21
88.87
132.05
12.34
3.78
2.29
84.12
103.53
27.60
12.13
3.00
84.16
126.88
28.00
17.85
3.93
80.65
130.38
1454.59
315.20
365.23
242.43
157.60
11.78
20.58
13.38
<0.01
<0.01
39.79
33.57
8.56
318.44
39.79
33.57
8.56
8.85
4.50
3.80
0.97
<0.05
NS
NS
39.37
7.85
4.03
81.61
39.27
7.85
4.03
2.54
17.25
3.05
1.85
<0.001
NS
NS
32.08
15.23
3.29
76.67
35.768
15.121
3.001
8.842
17.02
5.12
2.17
<0.001
<0.05
NS
29.80
14.82
3.00
76.01
27.168
13.102
2.925
11.240
16.50
4.99
2.10
<0.001
<0.05
NS
0.08
4.61
2.59
34.47
0.084
4.613
2.592
0.957
0.09
4.82
2.71
NS
NS
NS
29.06
5.30
4.00
66.17
29.001
5.295
3.999
6.615
9.75
4.23
3.78
<0.001
NS
NS
35.80
4.17
3.21
88.87
33.111
3.902
3.100
12.234
22.24
3.56
2.98
<0.001
NS
NS
12.34
3.78
2.29
84.12
9.223
3.548
2.290
9.121
11.23
3.34
2.00
<0.05
NS
NS
27.60
12.13
3.00
84.16
26.124
12.125
2.998
2.234
15.77
5.55
2.01
<0.001
<0.05
NS
39.77
15.78
3.93
80.65
39.768
15.784
3.925
2.240
17.75
7.05
1.75
<0.001
<0.05
NS
Dictyoptera
Lobopterella dimidatipes (Bolivar)
Pycnoscelus indicus (Fabricius)
Nematoda
Unidentified sp.
Annelida
Unidentified sp.
Mollusca
Gastrocopta tripunctata Morelet
Streptosele acicula Morelet
Opeas pumilum Pfeiffer
Araneae
Heliophanus activus (Blackwall)
O. aphanes (Thorell)
Oedignatha mogamoga Marples
Opopaea lena Suman
Schizomida
Ovozomus similis (Hirst)
Diplopoda
Dalodemidae sp.
Chilopoda
Crytops sp.
Psocoptera
Unidentified sp.
Isopoda
Trichorhina tomentosa (Budde-Lund)
Continued on next page
20
Table 1. Continued
Taxon
Dermaptera
E. annulipes (Lucas)
Gonolabis electra Burr
Diptera
(larvae)
Coleoptera
Anobiidae
Ptiniidae
Staphylinidae
Hymenoptera (Formicidae)
B. cordemoyi Forel
P. megacephala (Fabricius)
S. seychellensi Forel
Strumigenys emmae (Emery)
T. albipes (F. Smith)
T. lanuginosum Mayr
T. simillimum (F. Smith)
O. simillimus (F. Smith)
Variable
df
Seq. SS
Adj. SS
Adj. MS
F
P
Cover
Distance to sea
Anoplolepis
Error
Total
Cover
Distance to sea
Anoplolepis
Error
Total
Cover
Distance to sea
Anoplolepis
Error
Total
Cover
Distance to sea
Anoplolepis
Error
Total
1
1
1
36
39
1
1
1
36
39
1
1
1
36
39
1
1
1
36
39
17.11
7.22
2.20
88.97
115.49
36.00
17.00
2.99
90.16
146.15
1.47
0.97
0.75
10.78
13.98
5.49
2.08
2.89
18.92
29.38
17.11
7.22
2.20
88.97
15.232
6.889
2.194
12.240
11.23
3.05
1.85
<0.05
NS
NS
36.00
17.00
2.99
90.16
33.242
12.174
2.992
0.224
21.25
9.15
0.75
<0.001
<0.05
NS
0.76
1.16
0.75
10.78
0.758
1.161
0.751
0.299
2.53
3.88
2.51
9.45
1.54
2.89
18.92
9.452
1.538
2.888
0.526
17.98
2.93
5.49
<0.001
NS
<0.05
Cover
Distance to sea
Anoplolepis
Error
Total
1
1
1
36
39
2.33
1.89
363.52
109.67
477.42
2.12
1.32
363.52
109.67
2.123
1.320
11.982
2.987
5.33
3.03
23.23
<0.01
NS
<0.001
NS
NS
NS
Data are taken from all 40 grid squares in 2002. Native species are highlighted in bold.
Figure 3. The relationship between crazy ant activity at mid-day
and canopy cover (ant density ¼ 0.257 (canopy cover) – 18.545;
R2 ¼ 0.888; P < 0.001) in the visual sample quadrats in 2002.
9:30 h and between 16:30 and 18:00 h. During the
middle of the day, no ants were observed until the
leaf litter was disturbed when large numbers
emerged from under leaves and out of rotten logs.
The crazy ant was observed to forage on the
beaches at 17:00 h down to the water’s edge.
Night-time observations (18:00 to 23:30 h) in the
ant-infested areas revealed only small numbers of
foraging ants. No comparative measures of activity
could be obtained at night due to the difficulty of
making night-time observations without disturbing
the ants.
When active, ants were seen foraging on the surface and tending a species of aphid on Scaevola
sericea Vahl bushes. Ants were observed on
Pisonia grandis and Cordia subcordata Lam. trees
with heavy infestations of larvae of the indigenous
moth Ethmia nigroapicella Saalm€
uller. The larvae
were protected by silk webs and no direct predation
was observed. The ants were observed to feed on
the freshly broken surfaces of the leaves eaten by
the caterpillars. Items carried by crazy ants
included scale insects (Pulvinaria urbicola
Cockerell) and cydnid bugs. Freshly dead ghost
crabs (Ocypode ceratopthalma (Pallas)) were
observed on the northern beach, the large numbers
21
Table 2. Significant regressions predicting invertebrate abundance (derived from the data analysed in Table 1)
Cockroach
Spider
Ants
B. cordemoyi Forel
P. megacephala (Fabricius)
S. seychellensis Forel
S. emmae (Emery)
T. albipes (F. Smith)
T. lanuginosum Mayr
T. simillimum (F. Smith)
O. simillimus (F. Smith)
Beetles
Woodlice
Regression
t
P
Y ¼ 6.275–0.036 (cover)
Y ¼ 3.423–0.582 (sea)
Y ¼ 2.772–0.017 (cover) + 0.044 (Anoplolepis)
2.12
2.19
5.89
<0.05
<0.05
<0.001
Y ¼ 0.152 + 0.012 (cover)–0.122 (Anoplolepis)
None
Y ¼ 4.946–0.036 (cover)–1.077 (sea)
2.22
<0.05
4.21
<0.05
Table 3. Woodland composition and cover at points A and B
Number per hectares
A
B
Plant Habit
Plant species
Status
1999
2002
1999
2002
Open tree
Canopy tree
Canopy tree
Canopy tree
Shrub
% cover
Carica papaya
Casuarina equisetifolia
Cocos nucifera
Pisonia grandis
Scaevola sericea
Introduced crop plant
Native
Native (largely planted)
Native
Native
320
0
1120
640
0
90
310
0
1110
660
0
90
400
80
0
640
400
30
1240
60
0
720
300
70
of ants on these crabs and the unusually high density of fresh (<24 h old) remains (50 per hectare)
may be indicative of predation by the ants. Ten
nests of crazy ants contained remains of individuals
of eight earwigs (Euborellia annulipes (Lucas)), two
ants (Odontomachus simillimus), two crickets
(Zarceus fallaciosus Bolivar), one cockroach
(Periplaneta americana (Fabricius)) and one beetle
(Uloma sp.). In addition, live individuals of
E. annulipes and woodlice (Cubaris murina Brandt)
were found in crazy ant nests.
Biodiversity
No significant crazy ant-related invertebrate distribution patterns could be detected except for ants
and one species of paussine carabid beetle (the first
record of this ant parasite subfamily from
Seychelles) (Tables 1 and 2). The single species of
paussine beetle was positively associated with the
crazy ant whilst other ant species were reduced in
areas with abundant crazy ant. Most ant species
were widespread, but the largest species, O. simillimus
(F. Smith) did not coexist with crazy ants.
In the woodland area the mean level of crazy ant
activity at mid-day was 5.4 active ants per square
metre, with the highest density in the densest woodland. Leaf-litter samples (which also include ants
below the surface of the litter) contained over
10 times more ants (mean density of 60 m2). The
crazy ant was scarce in open woodland although it
does forage in open habitats at dusk. Differences in
woodland composition and cover in ant invaded
and ant-free areas are summarised in Table 3.
Active Lesser Noddy nests were found in ant
invaded and ant-free areas with no significant difference between the two areas (in the 10 trees
studied in each area ant-infested trees supported
49 nests, whilst ant-free trees had 45 nests).
Nest success was difficult to determine, but
approximately 50% of nests in both areas
22
contained visible chicks (49% in ant-infested trees,
51% in ant-free trees), suggesting that success was
comparable.
Areas free of the crazy ant were more exposed and
have a thin humus layer unsuitable for nesting.
Crazy ant behaviour
Discussion
Distribution of crazy ant
The spread of the crazy ant over Bird island since
1997 is estimated at 146–294 m per year, which is
considerably lower than the rate of 402 m per year
calculated on Mahe (Haines and Haines 1978a).
This may suggest that factors other than the intrinsic rate of colony expansion are important on Bird
island. Density in the invaded woodland on Bird
island (as indicated by leaf litter sampling with
Tullgren funnels) is approximately 0.6 million
ants per hectare (with a range of 0.08–0.79 million).
This is comparable with densities estimated for
parts of Mahe in 1978 (Union Vale 0.25–0.5 and
Mamelles 0.9 million) but is lower than the introduction focus on Mahe (1.3 million ha1; Haines
and Haines 1978b). The highest densities were in
the mature woodland, and no pattern of a high
density leading invasive edge could be detected.
This suggests that habitat characteristics are the
primary determinant of crazy ant abundance on
Bird island.
Woodland on Bird island has expanded considerably over the past 10 years. In 1994, it was
restricted to a small area of dense P. grandis,
spreading from about 5 ha in 1997 to its current
extent of approximately 70 ha (35 ha with permanent colonies). The expansion of the woodland in
the mid 1990s has resulted in improved conditions
for woodland-inhabiting invertebrates, including
the crazy ant. Their 2001 distribution reflects their
preference for shaded habitats, with colonies
restricted to the dense woodland. The woodland
that was not occupied by the crazy ant had a
much more open structure. Although the total
number of plants over 2 m ha1 was similar in
ant-free and ant invaded areas (1920 compared to
2160, respectively), the number of canopy forming
trees was 60% lower (720 compared to 1840). The
crazy ant-invaded areas were principally the overgrown coconut plantation, the abundant coconut
trees (1120 per hectare) providing dense shade and
also abundant rotten wood for nest construction.
The behavioural observations indicate that the
crazy ant is limited (spatially or temporally) by
high temperatures. It has been reported that
A. gracilipes activity is limited by temperature,
being restricted to temperatures in the range of
approximately 25–30 C (Dammerman 1929).
These are the normal shade temperatures experienced by the islands, although noon temperatures
above 30 C are frequent and in full sun temperatures would be expected to rise much higher.
Consequently, it is to be expected that crazy ant
activity will be reduced in the middle of the day and
that activity (particularly colony establishment)
will concentrate on shaded areas, such as the
woodland.
The crazy ant has been recorded preying on blind
snakes (Ramphotyphlops braminus Daudin), turtle
(Eretmochelys imbricata (Linnaeus)) hatchlings
and fairy tern (Gygis alba Sparrm) chicks that had
fallen from their nests (Robert 1998a; Feare 1999).
Disturbance to the sooty tern colony was noted in
1998 (Robert 1998b) ultimately resulting in the
terns abandoning part of the colony (Robert
1998c). The disturbance caused to the nesting
terns and their chicks appears to be principally
aggravation rather than direct predation. The ants
swarm over the ground and the terns, causing the
adults to preen frequently and the chicks to stamp
their feet, a behavioural mechanism that normally
serves to reduce tick infestations (Feare and Gill
1997). The swarming of the crazy ant results in the
tern chicks constantly stamping their feet, causing
excessive energy expenditure and reduced feeding,
ultimately resulting in exhaustion and death
(Robert 1999b). Tree-nesting terns (and possibly
other birds) appear to be able to coexist with
crazy ants, although there is a high degree of irritation caused by the ants running over the birds. At
present, there is no evidence that this irritation
causes high levels of mortality in tree-nesting terns.
Biodiversity
The impact of crazy ants on Bird island may be
highly significant as has been suggested in the past,
23
although previous reports have not distinguished
the impact of the ants from habitat effects, in particular some interpretations may be misleading due
to the close association between the ants and the
densest areas of vegetation. Reports of significant
impacts may reflect observations made at a different stage in the invasion process or temporary phenomena resulting from other, unrelated ecological
processes. This includes reports of die-back of
P. grandis caused by coccoid bugs P. urbicola cultured by crazy ants (Hill 2002). P. grandis die-back
is a temporary phenomenon and it may be too
simplistic to attribute this to a Pulvinaria–ant
interaction alone. Crazy ants do tend the coccoids
and infestations can be locally extremely high.
However, throughout 2001–2002, heavily-infested
trees remained healthy and any study of such dieback should also consider climatic variation.
Similarly reports of the decline of reptiles and
Polistes olivaceus (De Geer) wasps following the
crazy ant range expansion (Hill 2002) are not
supported by more recent observations.
The Tullgren funnel data indicate that crazy
ants do not have a significant effect on the abundance of invertebrates, with the exception of other
ant species and paussine beetles. All ant species
were positively correlated with crazy ant numbers
other than O. simillimus. The small ants may
share the microhabitat preferences of the crazy
ant but the large predatory O. simillimus is
excluded and directly fed upon by crazy ants. The
paussine beetles are generalist ant parasites and
their positive association with the crazy ant reflects
this parasitism. Similarly, the flowerpot snake
R. braminus was found to be locally abundant in
ant-infested areas and was observed feeding on
ants (P. megacephala).
The limited data available on crazy ant diet suggest that a large part of the faunal differences are
not due to direct predation by crazy ants. Haines
and Haines (1978a) identified Hymenoptera
(unspecified but implied to be mostly ants) as the
main food component (18.5% of food items), followed by cockroaches (15%), termites (11%), bugs
(11%), woodlice (10%), beetles (7%), plant material
(6%) and myriapods (5%). Other items totalled less
than 5%. Direct predation by crazy ants may be
expected to be most significant for other large ant
species (e.g., O. simillimus) and may be significant
for the large invertebrates apparently excluded by
the ants. Cockroaches may be expected to be
affected by crazy ants on this basis but the high
local densities of cockroaches in crazy ant habitat
may obscure any predation impacts. Alternatively,
the cockroaches may benefit from ants reducing
the numbers of other predators as appear to
be the case in agricultural habitat in La Reunion
(M. Samways, pers. commun.). It should also be
noted that in 2001, areas not invaded by crazy
ants were occupied by the native predatory ant
O. simillimus. This species is slightly larger than
the crazy ant and possesses powerful jaws. It is
likely to have as significant an impact on leaf-litter
inhabiting invertebrates as the crazy ant does. The
principal difference in the effects of these species is
that the colonies of O. simillimus are smaller and
more scattered and the species is predominantly
terrestrial, whilst crazy ants frequently forage on
vegetation (pers. observ.). A comparison of the
fauna of areas on Mahe with heavy crazy ant infestations with areas with few crazy ants by pitfall
trapping indicated that few other ant species coexisted with the crazy ant. Beetle, fly millipede, spider
and collembolan numbers were reduced, but that
cockroaches, hemipteran bugs, moths and woodlice were more abundant (Haines and Haines
1978a). These data are difficult to interpret, however, as unrecorded ecological factors may have
caused the differences in abundance in both the
crazy ants and the other invertebrates.
The current management of crazy ants on Bird
island comprises the spraying of insecticide around
habitations and the open area of the sooty tern
colony to keep the terns’ nesting area largely free
of ants during the nesting season. This affects only
the open areas and no management is carried out in
the woodland areas studied. Extension of the insecticidal spraying could result in eradication of the
ants but at potentially great cost to the invertebrate
fauna. The majority of the invertebrates are widespread species but there are some endemics of conservation significance (such as a new dalodemid
millipede species so far located only on Bird island).
The impact of crazy ants may be reduced if the ant
density declines. Such a decline can be expected in
future, as ant predators become more abundant
(principally the paussine beetles and the ant lion
Myrmeleon obscurus). Further reduction in crazy
ant abundance could be effected by habitat management. The current composition of the woodland
24
is heavily biased towards coconuts in crazy ant
areas. These provide ideal nesting places for the
ants and dense shade. The natural woodland
would originally have been dominated by
P. grandis, which has an open structure. If the
remnants of the old coconut plantation were eliminated, the density of trees were be reduced by 54%
and the tree canopy opened out by at least 30%.
This habitat structure would be less suitable for the
crazy ant than the current coconut dominated
habitat. The woodland is likely to become more
open naturally as the high rhino beetle population
kills the mature coconuts (mortality currently running at 5–10% annually), but the process could be
greatly enhanced by manual cutting of the young
palms.
The crazy ant is believed to have colonised Bird
island and all the Seychelles islands it inhabits as a
result of accidental introduction in supplies
imported from crazy ant infested islands (probably
Mahe in most cases). It is the most obvious introduced species on Bird island, although the invertebrate fauna include several introduced species. The
ant species include three native species (O. simillimus,
S. seychellensis Forel and Brachymyrmex cordemoyi
Forel) and four cosmopolitan species: Strumigenys
emmae (Emery). Tetramorium lanuginosum Mayr,
T. simillimum (F. Smith), Technomyrmex albipes
(F. Smith) (a tramp species of uncertain origin)
and P. megacephala. T. albipes is common in woodland and is present on both the ground and on
vegetation. P. megacephala and the Tetramorium
species are more restricted on Bird island and only
found on the ground. Both species are generally
regarded as introductions although their status in
the Seychelles fauna is currently uncertain (being
among the first ants recorded from the islands in
the 19th century). If these species are introduced it
is possible that any ant facilitated ecological
change has already taken place, obscuring any
impacts of crazy ant invasion.
Invasive ants have been reported to disrupt
entire ecosystems (Porter and Savignano 1990;
Vinson 1991) with direct impacts on plants (Bond
and Slingsby 1984) and other ant species (Clark
et al. 1982; de Kock and Giliomee 1989; Brand~ao
and Pavia 1994; Holway 1995; Human and Gordon
1996). Of these direct impacts, it has been found
that ant species categorised as non-cryptic aboveground foragers are most significantly affected
(Holway 1995; Human and Gordon 1996). The
exclusion of the large surface feeding O. simillimus
by A. gracilipes on Bird island is in accordance with
this finding. The findings of this study suggest that
the impacts of invasion by introduced species may
be obscured by the ecological processes of the biological system concerned and habitat effects, especially when there is a significant change in the
habitat. This results in the frequently confusing
picture of alien ants coexisting with native species
and unpredictable outbreaks of invasives. This
complexity of species change has long been recognised (Haskins and Haskins 1965; Wilson and
Taylor 1967), although rarely reported in recent
reviews of ant invasions.
Acknowledgements
I am grateful to Guy Savy and Serge Robert for
arranging my visits to Bird island and for extensive
discussions on the impacts of crazy ants. I am also
grateful for the highly constructive comments of an
anonymous reviewer.
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